If your Maxtor external was formatted NTFS for Windows, then there is no solution, except to erase and reformat in a usable format for the Mac. Microsoft just does not give out any info for writing/modifying files on an NTFS drive, from a non-Windows operating system. This results in not being able to write to the drive, or modify any files on that drive, when you are accessing the drive from a Mac.
Here's your help - Open your Disk Utility (in your Applications/Utilities folder)
Click on the hard drive to select it. Should be a line with the name Maxtor at the beginning.
Click the Partition tab.
Click Current (under Volume Scheme), and change to 1 partition
Make sure that MacOS Extended is showing in the Format line (MacOS Extended (Journaled) is OK, but not needed)
Click the Partition button at the bottom of that window.
When that completes, eject the drive, and disconnect the cable, turn the drive power off.
Turn back on, plug the cable back in, and you should be OK.
note: You will lose EVERYTHING that is presently stored on your external drive when you do this. There is no other way to do this, because of the NTFS partition. Once complete, you will not be able to use this drive on a PC, unless you use MacDrive software on the PC, or something similar.
If you don't want to lose anything - then purchase a second drive to use on the Mac only. You could also choose to format the drive as Fat32, and put up with the possibility of occasional data loss, and the inability to copy really large files. You will still lose all files on the drive, even if going to Fat32.